<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>060900 - Disclosure - PREFERRED STOCK</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>1</Level><ElementName>us-gaap_EquityAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>PREFERRED STOCK [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_PreferredStockTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20130101to20130630" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; font-weight: bold; text-decoration: underline; display: block; text-indent: 0pt;"&gt;NOTE 9 &amp;#8211; PREFERRED STOCK&lt;/div&gt;&lt;div style="text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;During 2012, the Company entered into an agreement with its largest stockholder, Brightline Ventures I, LLC (&amp;#8220;Brightline&amp;#8221;), pursuant to which Brightline agreed to convert $3,326,697 (exclusive of dividends)&lt;font style="font-weight: bold; display: inline;"&gt;&amp;#160;&lt;/font&gt;worth of Series II Preferred Stock into 3,326,697&lt;font style="font-weight: bold; display: inline;"&gt;&amp;#160;&lt;/font&gt;shares of the Company&amp;#8217;s Common Stock at the earlier of either the maturity date of the security or the closing of any approved transaction with Maxim Group, LLC.&amp;#160;&amp;#160;In consideration for the foregoing conversion of Series II Preferred Stock by Brightline on or before their respective maturity dates, the Company modified the following warrants held by Brightline to provide them with the ability to exercise such warrants on a cashless basis: (i) warrants to purchase an aggregate of&amp;#160;&amp;#160;11,582,983&lt;font style="font-weight: bold; display: inline;"&gt;&amp;#160;&lt;/font&gt;shares of Common Stock with an exercise price of $1.50 per share, which were issued to Brightline in transactions where Brightline acquired shares of the Company&amp;#8217;s Series I and II Preferred Stock; and (ii) warrants to purchase an aggregate of&amp;#160;&amp;#160;2,859,375 shares of Common Stock with an exercise price of $1.50 per share, which equaled one half of the outstanding&amp;#160;&amp;#160;and unexercised warrants issued to Brightline in other transactions where Brightline provided financing to the Company.&amp;#160;&amp;#160;As a result of this modification, the Series II Convertible, Redeemable Preferred Stock was reclassified on the balance sheet as permanent Stockholders&amp;#8217; Equity and identified as Series II Convertible Preferred Stock at December 31, 2012. The value reclassified to permanent equity was equal to the carrying value of the shares on the date the agreement was modified. This value was $2,157,238 based on a gross value of $3,326,697 and a remaining discount of $1,169,459. Furthermore, the accrued dividends relating to the Series II Convertible Preferred Stock were also reclassified to Stockholders&amp;#8217; Equity as of December 31, 2012.&amp;#160;&amp;#160;These reclassifications were necessary based on the preferred shares no longer being subject to potential cash redemption.&lt;/div&gt;&lt;div style="text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;On March 18, 2013, Brightline converted the Series II Convertible Preferred Stock of $3,326,697 together with $533,000 of accrued dividends thereon into 3,859,697 shares of the Company&amp;#8217;s Common Stock.&amp;#160;&amp;#160;As of March 31, 2013 there was no Series II Convertible Preferred Stock outstanding.&amp;#160;&amp;#160;The value of the Series II Preferred Stock and dividends converted was equal to the total value recorded to common stock and additional paid in capital with no gain or loss recorded as the conversion was consistent with the original agreement.&amp;#160;&amp;#160;As a result of the preferred stock conversion, $2,690,056 was recorded to common stock and additional paid-in capital.&amp;#160;&amp;#160;There were no accrued dividends at June 30, 2013; accrued dividends were $476,857 as of December 31, 2012.&lt;/div&gt;&lt;div style="text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;As of June 30, 2013, no shares of Series I or II Preferred Stock remained outstanding.&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for terms, amounts, nature of changes, rights and privileges, dividends, and other matters related to preferred stock.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>PREFERRED STOCK</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>PREFERRED STOCK</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://ztrim.com/role/PreferredStock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
